The melody is usually the line that is prominently superseding all other voices.  It can be heard by itself, and it makes perfect sense.  Within that melody you have phrases, not unlike an independent clause in a sentence.  A melody line is considered one voice.  When you add other voices to that melody, the other voices compliment the melody.  This is done either by adding other independent melodies, or through the use of chords.  A lot of pop songs, for example, use the simple technique of supporting a prominent melody/singer with chords.

Two separate conversations can be very hard to follow.  You will usually follow one voice or the other, and you may eventually become annoyed.  In music, however, two voices can compliment one another.  They can be, at times, in harmony with one another.  At other other times there can moments of dissonance that sound sweet when it resolves or moves towards harmony.  The way a melody or even multiple voices move is called voice leading.  Voice leading is a way for multiple voices to move through a phrase.  Proper voice leading will speak in a way that leads the listener from one place to another.

I like the use of multiple melodies in a song.  At the same time a strong chord progression is key to establishing a sense of direction.  I enjoy a song that can take you on a journey of sorts.  Music for video or film is fashioned to do just that, but when music can take you “somewhere” without it already being connected to a film or video, that is something special.